A free-piston (“FP”) engine is a “crank-less” internal combustion engine in which the piston moves within an elongated chamber by a combustion within the combustion portion of the chamber. In conventional crank internal combustion (“IC”) engines, the piston is connected to a flywheel by a linear crankshaft such that the linear movement of the piston from the combustion event actuates the crankshaft to rotate the flywheel for operating a hydraulic pump or other mechanical system. In conventional IC engines, the continued rotation of the flywheel is translated to the piston via the crankshaft to cycle the piston back to the original position. In contrast, the expansion of the piston of the FP engine operates a hydraulic pump, linear alternator or other load device to store or use the kinetic energy. In certain FP engines, the exhaust gases from the combustion event are also fed through a gas turbine engine. The piston of the FP engine is compressed by using a portion of the stored energy to reverse the load device or with a rebound device, such as an opposing free-piston engine.
The absence of a crankshaft and flywheel assembly in an FP engine reduces the number of moving parts thereby reducing frictional losses in the load device from the moving parts providing improved efficiency of the FP engine. Without the crankshaft and flywheel, the cycling of the FP engine is mainly dependent on the dynamic coupling of the in-cylinder gas dynamics, the load applied by the load device and the piston trajectory. However, unlike conventional IC engines where the crankshaft can be used to correct irregular movement, FP engines cannot directly mechanically control the piston movement. As a result, the operation of FP engines often varies cycle-to-cycle, especially during transient operation such as combustion events, which can make engine control difficult and cause the engine to misfire. In particular, the FP engine is subject to transient behavior when switching between operational modes such as from motoring mode to a firing mode and vice versa resulting in large tracking errors and other ill effects.
The transient nature of FP engines makes achieving robust and precise engine operation control difficult. The current control methodologies for FP engines are primarily calibration methodologies that have had limited success and are primarily limited to single piston FP engines. In a calibration-based methodology, the system is set for normal operating mode based on desired operation conditions and at an effective efficiency. However, transient events can create irregular piston trajectory that cannot be efficiently regulated by current calibration-based methodology.